The present invention relates to optical medical sensors, especially oximetry sensors and, in particular, pulse oximetry sensors which include coded information relating to characteristics of the sensor.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient""s tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
The intensity of the light emitters in the sensor, typically LEDs, can be controlled by controlling their drive current. Generally, it is advantageous to have a higher intensity level to increase the signal to noise ratio of the signal received at the detector. The amount of drive can vary depending upon the characteristics of the particular LEDs and detector. Thus, some oximeters will vary the drive current to the LEDs in accordance with the signal received from the detectors until an optimum level has been obtained. In particular, the drive current may vary between a red and an infrared LED, with different drive levels required for the different types of LEDs to ensure that both fall within the range of an analog-to-digital converter connected to the amplified detector signal. One limitation on the amount of drive current is that too high a current could cause the LEDs to emit sufficient heat to burn the patient. Accordingly, a maximum current allowable is often imposed, and is chosen to be conservative taking into consideration the variations in LED performance from LED to LED, and also taking into consideration the different heat-dissipating properties of the tissue of different patients to whom the sensors may be connected.
An example of an encoding mechanism not related to controlling drive current is shown in U.S. Pat. No. 4,700,708. This relates to an optical oximeter probe which uses a pair of light emitting diodes (LEDs) to direct light through blood-perfused tissue, with a detector picking up light which has not been absorbed by the tissue. The operation depends upon knowing the wavelength of the LEDs. Since the wavelength of LEDs actually manufactured can vary, a resistor is placed in the sensor with the value of the resistor corresponding to the actual wavelength of at least one of the LEDs. When the instrument is turned on, it first applies a current to the coding resistor and measures the voltage across the resistor to determine the value of the resistor and thus the value of the wavelength of the LED in the probe.
U.S. Pat. No. 4,913,150 recognizes that the coded value of the wavelength of the red LED provided by a coding resistor may be inaccurate, since the actual wavelength can vary with temperature. Accordingly, this patent teaches including a temperature sensor in the oximeter sensor to measure the actual temperature. With the actual temperature, and the coded wavelength value, a look-up table can be consulted to determine the actual LED wavelength for that temperature.
Another method of storing coded information regarding the wavelength characteristics of the LEDs is shown in U.S. Pat. No. 4,942,877. This patent discloses using an EPROM memory on the sensor to store digital information, which can be provided in parallel or serially from the sensor probe to a remote oximeter.
Other examples of coding sensor characteristics exist in other areas. In U.S. Pat. No. 4,446,715, assigned to Camino Laboratories, Inc., a number of resistors are used to provide coded information regarding the characteristics of a pressure transducer. U.S. Pat. No. 3,790,910 discloses another pressure transducer with a ROM storing characteristics of the individual transducer. U.S. Pat. No. 4,303,984 shows another sensor with digital characterization information stored in a PROM, which is read serially using a shift register. U.S. Pat. No. 5,651,780 shows a catheter having means for generating a uniquely coded identification signal. This signal may contain a variety of information, including the presence of a temperature sensor or thermistor and its associated resistance calibration, and the catheter product number.
Typically, the coding element is mounted in the sensor itself. For instance, U.S. Pat. No. 4,621,643 shows the coding resistor mounted in the sensor element itself. In addition, U.S. Pat. No. 5,246,003 shows the coding resistor being formed with a printed conductive material on the sensor itself.
In some devices, an electrical connector coupled by a cable to a device attached to a patient may include a coding element. For example, U.S. Pat. No. 3,720,199 shows an intra-aortic balloon catheter with a connector between the catheter and the console. The connector includes a resistor with a value chosen to reflect the volumetric displacement of the particular balloon. U.S. Pat. No. 4,684,245 discloses a fiberoptic catheter with a module between the fiberoptic and electrical wires connected to a processor. The module converts the light signals into electrical signals, and includes a memory storing calibration signals so the module and catheter can be disconnected from the processor and used with a different processor without requiring a recalibration.
The present invention provides a method for operating an oximeter sensor, and corresponding apparatus, which includes in the sensor, or in a sensor attachment (e.g., connecting cable or plug), an element such as a light emitter, having a temperature-dependent electrical characteristic. The temperature dependence of the electrical characteristic is encoded in a component of the sensor assembly. The encoded temperature characteristic is read, and is used to modify the driving of the light emitter in the sensor. This enables a light emitter to be operated at its maximum allowable intensity to maximize the signal to noise ratio, without burning a patient, in accordance with the particular characteristics of that light emitter.
In one embodiment, the temperature at the patient""s skin can be estimated from the junction temperature of an LED contained in a pulse oximeter sensor. The junction temperature can be measured by driving the junction with a fixed current and measuring the corresponding forward voltage drop. The peak skin temperature under the LED can be estimated computationally from the junction temperature, the power dissipated in the sensor LEDs, and the effective thermal conductivities of the sensor body and of the patient.
In another embodiment of the invention, the junction temperature of an LED incorporated in a medical optical sensor may be determined in order to compute the shift in principal wavelength of the LED. This allows a more accurate computation of the shift than has previously been possible in LED-based medical optical sensors. The wavelength-shift information in turn may be used to select or compute a calibration curve for determining arterial oxygen saturation for the sensor with higher accuracy than would otherwise have been possible.
In another aspect of the present invention, the voltage across a sensor element for given drive current is measured, and compared to a calibration value. A variance, especially one indicating a higher resistance, can indicate worn or damaged connections, while a lower resistance could indicate a short-circuit condition. Thus, an error signal indicating a defective sensor can be automatically produced.
In still another aspect of the invention, the apparent forward voltage across an element (e.g., an LED) in the sensor is measured at two different currents. A dynamic resistance value of the element is provided by one of several means, and is used in combination with the two measured forward voltages to determine the series resistance between the instrument and the element. This permits the actual forward voltage across the element to be determined more accurately, with a resulting enhancement of temperature measurement accuracy.
For further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.